Optical transmission equipment and optical add-drop multiplexer

ABSTRACT

An optical add-drop function part of an optical add-drop multiplexer has an output end of transmitted light signal in which an optical termination mechanism formed by an optical detector for detecting open of an optical fiber, an optical switch, and an optical terminator are mounted. Further, a reflection level calculation circuit, a reflection warning determination circuit, and an optical switch selection circuit are mounted to perform laser safety in the optical add-drop function part as well. The laser safety part in the optical add-drop function part is operated earlier than the laser safety part in an optical amplification function part.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent applicationserial no. 2006-173915, filed on Jun. 23, 2006, the content of which ishereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to optical transmission equipment and anoptical add-drop multiplexer (OADM) that use a wavelength multiplexingtechnique. More particularly, the invention relates to opticaltransmission equipment and an optical add-drop multiplexer that haveexcellent maintainability.

With the increasing capacity of data system communication represented bythe Internet technology, a rapid growth in data volume and an increasein the associated transmission capacity are expected in opticaltransmission systems. In order to meet such demands, the wavelengthmultiplexing technique is applied to perform communication by bundlingplural signal lights of different wavelengths through one optical fiber.Recently, there is being established a communication network usingoptical add-drop multiplexers that can drop and add optical signals foreach wavelength at plural locations, in addition to transmit largevolume of data between two separate sites by means of the wavelengthmultiplexing technique.

FIG. 1 shows a block diagram of an optical transmission network usingoptical add-drop multiplexers. An optical transmission network 1000includes six optical add-drop multiplexers 101 that are connected in aring configuration through an optical fiber transmission line 102. Theoptical add-drop multiplexer 101 selects whether to add and drop pluraloptical signals of different wavelengths for each wavelength, or whetherto transmit the optical signals through the multiplexer. In FIG. 1,there are shown the start points and endpoints of five optical signalsλ1 (lambda 1) to λ5 each having different wavelengths, all of which areadded and dropped at nodes that can be freely selected.

FIG. 2 is a block diagram showing a principal part of the opticaltransmission network. FIG. 2 shows the device configuration of a portionof the optical network shown in FIG. 1, from an optical add-dropmultiplexer 101-1, to an optical add-drop multiplexer 101-2, and to anoptical add-drop multiplexer 101-3. However, in FIG. 2, there are onlyshown an East side function part for the optical add-drop multiplexer101-1 and a West side function part for the optical add-drop multiplexer101-3. The optical add-drop multiplexer 101 includes an opticalamplification function part (West) 202-2, an optical amplificationfunction part (East) 202-1, an optical add-drop function part (West)201-2, and an optical add-drop function part (East) 202-1. The opticalamplification function part 202 includes: a reception optical amplifier203 for amplifying an input optical signal from the optical fibertransmission line 102 and transmitting the amplified signal to theoptical add-drop function part 201; and a transmission optical amplifier204 for amplifying an input optical signal from the optical add-dropfunction part 201 and transmitting the amplified signal to the opticalfiber transmission line 102. The optical add-drop function part 201includes: an optical drop part having an optical coupler 206-2 and anoptical demultiplexer 207; and an optical transmission/add selectionpart having the optical demultiplexer 207, an optical multiplexer 208,an optical switch 209, and a variable optical attenuator (VOA) 210.

Taking an example of the optical signal flow in a direction from West toEast in the optical add-drop multiplexer 101-2, the operation of theentire optical add-drop multiplexer will be described. A receivedoptical signal from the optical add-drop multiplexer 101-1 is amplifiedby the reception optical amplifier 203 of the optical amplificationfunction part (West) 202-1 of the optical add-drop multiplexer 101-2.Then the amplified signal is transmitted to the optical add-dropfunction part (West) 201-2. Incidentally, the operations of an opticalcoupler 206-1 and a laser safety part 205 will be described below withreference to FIG. 4. In the optical add-drop function part (West) 201-2,the optical signal is split into two halves by the optical coupler206-2, one of which is further branched into lights at each wavelengthby the optical demultiplexer 207 and is output from a drop optical port260-2. The other optical signal is transmitted as it is to the opticaladd-drop function part (East) 201-1 through an optical fiber 211connecting the optical add-drop function parts. In the optical add-dropfunction part (East) 201-1, the optical signal is branched into opticalsignals at different wavelengths by the optical demultiplexer 207, andthe signals are input to the optical switch 209. The optical switch 209selects and outputs either the transmitted optical signal from West orthe added optical signal from an add optical port 250-2 of the opticaladd-drop multiplexer 101-2. The variable optical attenuator 210 isprovided in the later stage of the optical switch 209 to equally adjustall the optical power levels of each of the wavelengths. The light whoseoptical power levels are adjusted by the variable optical attenuator 210is wavelength multiplexed by the optical multiplexer 208, and istransmitted to the optical amplification function part (East) 202-1. Inthe optical amplification function part (East) 202-1, the wavelengthmultiplexed light is amplified by the transmission optical amplifier 204and is transmitted to the optical fiber transmission line 102.

FIG. 3 is a view illustrating the optical signal flow from the opticaladd-drop multiplexer 101-1, to the optical add-drop multiplexer 101-2,and to the optical add-drop multiplexer 101-3 in the optical network ofFIG. 1. The optical signal λ1 is dropped and added in the opticaladd-drop function part (East) 201-1 of the optical add-drop multiplexer101-1 and in the optical add-drop function part (West) 201-2 of theoptical add-drop multiplexer 101-3, while being transmitted through theoptical add-drop multiplexer 101-2. Similarly, the optical signal λ2 isdropped and added in the optical add-drop multiplexers A, B. The opticalsignal λ3 is transmitted through the optical add-drop multiplexer 101-1,while being dropped and added in the optical add-drop multiplexer 101-2.The optical signal λ4 is dropped and added in the optical add-dropmultiplexer 101-2, while being transmitted through the optical add-dropmultiplexer 101-3.

In FIG. 2, the reception optical amplifier 203 of each of the opticalamplification function parts 202-1, 202-2 has a function of compensatingthe optical power reduction including not only a loss in the opticalfiber transmission line but also a loss in the optical add-drop functionpart. Consequently the optical power level is high. Assuming that theoptical power level for one wavelength is +6 dBm in the receptionoptical amplifier 203, the optical power level for 40 wavelengthsreaches +22 dBm which corresponds to a laser standard class 3B definedby JIS C 6082. There is a risk that the eyes will remain damaged bydirectly seeing such a laser beam. In order to avoid such a risk, thereception optical amplifier 203 includes a laser safety function forautomatically reducing the optical power level to about an optical powerlevel at one wavelength (about +5 dBm or less) which corresponds to aclass 1 standard, by detecting an output open of the optical fiber byreflected light. In FIG. 2, the laser safety function is realized usingthe optical coupler 206-1 and the laser safety part 205. The lasersafety function in the optical amplifier as described above is disclosedin JP-A No. 200130/1997, JP-A No. 144687/2001, and JP-A No. 335214/2002.

In the configuration of FIG. 2, it is assumed that a failure occurs inthe optical add-drop function part (East) 201-1 of the optical add-dropmultiplexer 101-2 and the relevant function part is needed to bereplaced. In this case, main signal interruption occurs in the twosignals λ1, λ4 of the optical signals shown in FIG. 3, and main signalinterruption should not occur in the optical signals λ2, λ3 that areoriginally not involved in the replacement. However, when the opticalfiber 211 connected between the optical add-drop function part (West)201-2 and the optical add-drop function part (East) 201-1 is removed,the laser safety part 205 detects an output open of the optical fiber byreflected light, thereby providing laser safety to the reception opticalamplifier 203. Given the optical level per wavelength of +6 dBm in theoutput of the reception optical amplifier 203, the reception opticalamplifier 203 of the optical amplification function part (West) 202-2amplifies the three signals of λ1, λ2, λ3, so that the total opticalpower level of all the optical signals is +10.8 dBm. This will bereduced to +5 dBm because the laser safety functions due to removal ofthe optical fiber connected between the optical add-drop function parts.In other words, the optical power level per wavelength is reduced by 4.8dB to +1.2 dBm, which has an impact on main signal continuity of theoptical signals λ2 and λ3, causing main signal interruption. The abovedescription has been made on the optical signals of three wavelengths.However, assuming that the present system is a system supporting 40wavelengths, the optical level is reduced by up to 16 dB according tothe calculation in the same way as described above.

The simplest way to solve the above problem is to insert an opticalisolator into an input end from the reception optical amplifier of theoptical add-drop function part. In this case, however, the laser safetydoes not function because the optical fiber is opened during thereplacement of the optical add-drop function part. From the output ofthe reception optical amplifier to the fiber connected between theoptical add-drop function parts, the optical loss occurs only in theoptical isolator and the optical coupler, and their loss is at mostabout 2 dB in total. In the case of 40-wavelength system, the eyes maybe damaged by directly seeing a maximum of +20 dBm during removal of theoptical fiber 211 between the optical add-drop function parts. For thisreason, in the method of inserting the optical isolator, it is necessaryto have a structure in which a light blocking function such as anoptical fiber connection shutter is provided to prevent the eyes fromdirectly seeing the light from the optical fiber.

SUMMARY OF THE INVENTION

The present invention provides optical transmission equipment and anoptical add-drop multiplexer that have no impact on a main signalwithout being involved in a maintenance operation for packagereplacement necessary due to a failure in the optical add-dropmultiplexer.

The above described object can be achieved by optical transmissionequipment including an optical amplification function part and anoptical drop function part. The optical amplification function partincludes: an optical amplifier connected to a transmission line toamplify the optical signal received from the transmission line; and afirst laser safety part for controlling the optical amplifier based onthe power of output light of the optical amplifier and on the power ofreflected light to the first optical amplifier. The optical dropfunction part includes: an optical coupler connected to the opticalamplification function part to split the optical signal received fromthe optical amplification function part; an optical demultiplexer forwavelength demultiplexing a first output optical signal of the opticalcoupler; a reflected light mask part for receiving a second outputoptical signal of the optical coupler as an input; and a second lasersafety part for controlling the reflected light mask part based on thepower of output light of the reflected light mask part and on the powerof reflected light to the reflected light mask part.

Further the above object can be achieved by an optical add-dropmultiplexer including: an optical amplification function part; anoptical drop function part; an optical add function part; and a secondoptical amplifier connected to the optical add function part to amplifyan optical signal received from the optical add function part. Theoptical amplification function part includes: a first optical amplifierconnected to a transmission line to amplify an optical signal receivedfrom the transmission line; and a first laser safety part forcontrolling the first optical amplifier based on the power of outputlight of the first optical amplifier and on the power of reflected lightto the first optical amplifier. The optical drop function part includes:an optical coupler connected to the optical amplification function partto split the optical signal received from the optical amplificationfunction part; a first optical demultiplexer for wavelengthdemultiplexing a first output optical signal of the optical coupler; areflected light mask part for receiving a second output optical signalof the optical coupler as an input; and a second laser safety part forcontrolling the reflected light mask part based on the power of outputlight of the reflected light mask part and on the power of reflectedlight to the reflected light mask part. The optical add function partincludes: a second optical demultiplexer connected to the optical dropfunction part to wavelength demultiplex the optical signal received fromthe optical drop function part; plural optical switches for receivingthe outputs of the second optical demultiplexer and receiving from anadd optical port, as inputs; and an optical multiplexer for wavelengthmultiplexing the outputs of the plural optical switches.

Further the above object can be achieved by an optical add-dropmultiplexer including: a reception optical amplification part includinga first optical amplifier for amplifying received wavelength multiplexedlight; an optical drop part for wavelength demultiplexing a portion ofthe wavelength multiplexed light; an optical add part for replacing thewavelength demultiplexed signals of the other portion of the wavelengthmultiplexed light; and a transmission optical amplification partincluding a second optical amplifier for amplifying the wavelengthmultiplexed light from the optical add part. The reception opticalamplification part has an output portion provided with a first lasersafety part for controlling the first optical amplifier by monitoringreflected light. Further the optical drop part has an output portionprovided with a reflected light mask part for monitoring and maskingreflected light, and with a second laser safety part for controlling thereflected light mask part.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described inconjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of an optical transmission network using anoptical add-drop multiplexer;

FIG. 2 is a block diagram showing a principal part of the opticaltransmission network;

FIG. 3 is a view illustrating the main signal flow in the opticaltransmission network;

FIG. 4 is a block diagram showing a principal part of an opticaladd-drop multiplexer;

FIG. 5 is a time chart illustrating the device operation when atransmission optical fiber of an optical add-drop function part isremoved; and

FIG. 6 is a block diagram showing a principal part of the opticaladd-drop multiplexer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Modes for carrying out the invention will be described below based onpreferred embodiments with reference to the accompanying drawings. Likeparts are given like reference numbers and their description will not berepeated.

Embodiment 1

A first embodiment will be described with reference to FIGS. 4 and 5.Here, FIG. 4 is a block diagram showing a principal part of an opticaladd-drop multiplexer. FIG. 5 is a time chart illustrating the deviceoperation when a transmission optical fiber of an optical add-dropfunction part is removed.

FIG. 4 shows an optical amplification function part and an opticaladd-drop function part, both only on West side in the optical add-dropmultiplexer. As for optical parts, an optical switch 209, an opticalcoupler 206-3, a reflected light monitoring optical detector 401-3, anoutput light monitoring optical detector 401-4, and an opticalterminator 406 are added to a transmitted light output end of an opticaladd-drop function part 201-2. Here, the optical coupler 206-3 splits theoptical signal to the optical fiber 211 and to the optical detector401-4, while splitting the reflected light to the optical switch 209 andto the optical detector 401-3. The optical terminator 406 is an opticalattenuator.

In the optical add-drop function part 201-2, the optical signal isconverted by the output light monitoring optical detector 401-4 into anelectrical signal that is proportionate to the output light power fromthe optical add-drop function part 201-2, and the optical signal isconverted by the reflected light monitoring optical detector 401-3 intoan electrical signal that is proportionate to the reflected light powerupon removal of the optical fiber 211 connected between the opticaladd-drop function parts. Then the amount of reflection is calculated ina reflection level calculation circuit 402-2 by the monitor results ofthe optical detectors. The reflection level calculation circuit 402-2performs the following calculation:Amount of reflection=Reflected light power/Output light power . . .  (1)If no output light power is given, the calculation circuit outputs acertain fixed amount of reflection without performing the abovecalculation process. This is to continue warning even when the outputlight power is absent. A reflection warning determination circuit 403-2periodically monitors the calculation result of the calculation circuitat an interval of 1 ms, and transmits a reflection warning to an opticalswitch selection circuit 405 when the amount of reflection exceeds areference value for a predetermined time period. Upon detection of thewarning, the optical switch selection circuit 405 performs switching ofthe optical switch 209 so that the optical signal proceeds to theoptical terminator 406. The principle of reflection warning detection inthe reception optical amplifier 203 of the optical amplificationfunction part 202-2 is exactly the same as in the case of the opticaladd-drop function part. The laser safety circuit 205 of the receptionoptical amplifier 203 is different from the laser safety circuit of theoptical add-drop function part 201-2, in only a point that an opticalamplifier control circuit 404 is provided instead of the optical switchselection circuit 405. The reception optical amplifier 203 controls theoutput power to be constant by reducing the output of the opticalamplifier to a safe level by the optical amplifier control circuit 404,instead of switching the optical switch upon detection of the reflectionwarning due to removal of the fiber.

In FIG. 5, (a) shows the state of the output fiber of the opticaladd-drop function part, (b) shows the calculation result of thereflection calculation circuit, (c) shows the output of the reflectionwarning determination circuit, (d) shows the control signal output ofthe optical switch control circuit, (e) shows the transmitted lightoutput level of the optical switch part, (f) shows the calculationresult of the reflection calculation circuit of the opticalamplification function part, (g) shows the output of the reflectionwarning determination circuit, (h) shows the control signal output ofthe optical amplifier control circuit, and (i) shows the output level ofthe optical amplifier.

In FIG. 5, the optical fiber 211 connecting the optical add-dropfunction parts is removed at time 0. At the same time of the removal ofthe optical fiber 211, the calculation result of the reflectioncalculation circuit of the optical add-drop function part and thecalculation result of the reflection calculation circuit in the opticalamplification function part are raised to their reflection detections,respectively. After time t1 when the reflection calculation result ofthe optical add-drop function part 201-2 reaches the reflection warningdetection level, the warning detection is actually determined by thereflection warning determination circuit 403-2. Here, t1 represents aprotection time. Similarly, time t3 represents a protection time fromwhen the reflection calculation result of the optical amplificationfunction part 202-2 reaches the reflection warning determination levelto when the warning detection is actually determined. Here, t1 issufficiently smaller than t3. Time t2 represents a time when theswitching of the optical switch is completed in the optical add-dropfunction part 201-2 and when the optical signal is terminated in theoptical terminator. Accordingly the value of t2-t1 is an actualswitching time on the optical switch hardware. Incidentally t1 and t3are set by a timer of the reflection warning determination circuit 403.

When the optical fiber is removed at time 0, the optical add-dropfunction part 201-2 and the optical amplification part 202-2 detect atsubstantially the same time that the reflection calculation resultreaches the reflection warning detection level in the reflection levelcalculation circuit 402. The optical add-drop function part 201-2 andthe optical amplification function part 202-2 operate differently afterthe refection calculation. The optical add-drop function part 201-2transmits a reflection warning by the reflection warning determinationcircuit 403-2 after the protection time t1 when the reflection resultreaches the reflection warning detection level, and then moves to lasersafety operation by optical switch selection operation. On the otherhand, the optical amplification function part 201-2 does not detect thereflection warning because the reflection calculation result does notreach the protection time t3 at the time when the protection time t1 haspassed, and remains in the normal mode of operation without moving tolaser safety operation to the optical amplifier. At time t2 when theoptical switch selection operation is completed, the optical add-dropfunction part 201-2 is still detecting the reflection warning. However,the optical amplification function part 202-2 does not detect thereflected light as the light is terminated in the optical add-dropfunction part 201-2, in which the reflection calculation result is belowthe reflection warning detection level. At time t2 when the reflectioncalculation result does not reach the protection time t3 of thereflection warning detection in the optical amplification function part202-2, the optical amplification function part continues normaloperation without entering at all the laser safety operation which isperformed in response to the detection of reflected light.

Incidentally, when the transmitted light level in the optical switchpart is reduced to the safe level and the output light power is “0” inthe reflection calculation circuit of the optical add-drop functionpart, the reflection calculation circuit outputs a certain fixed amountof reflection. As a result, the optical add-drop function part continuesto detect the warning.

In the case of using a mechanical optical switch as the optical switch209, the switching time, t2-t1, in the optical switch 209 is about 1 ms.Assuming that t1 is 5 ms, the time needed for the optical add-dropfunction part 201-2 to move to the laser safety operation is 6 ms.Accordingly the protection time t3 of the reflection warning detectionin the optical amplification function part 202-2 can be determined to 60ms which is ten times larger than 6 ms. Incidentally 60 ms is asufficiently short time as an exposure time of the eyes to the class 3Blaser.

By applying the embodiment to the optical add-drop multiplexer 101-2 inthe network configuration shown in FIG. 2, even when the optical fiber211 connecting the optical add-drop function parts is removed forpackage replacement necessary due to a failure in the optical add-dropfunction part (East) 201-1, the maintenance operation can be performedwithout having any impact on the optical signals λ2, λ3 that areoriginally not involved in the package replacement because the opticalamplification function part (West) 202-2 does not enter the laser safetyoperation. In addition, there is no risk that the eyes will be damagedby directly seeing the optical fiber as the transmitted light output ofthe optical add-drop function part (West) 202-2 is terminated by theoptical switch 209. However, once the optical add-drop function part202-2 enters the laser safety, the light is perfectly terminated and thereflection warning recovery will not be detected any more. For thisreason, it is necessary to return the optical switch to the normal lineby an operator operation when the maintenance operation is completed.

Incidentally FIG. 2 shows an example of the two-way network, but theembodiment can be applied to transmission equipment for one-waytransmission lines. This is the same in a second embodiment. The opticalswitch 209 and the optical terminator 406 can be a reflected light maskpart for preventing the reflected light from being seen by the opticalamplification function part 202-2.

According to the embodiment, the laser safety can be operated solely bythe optical add-drop function part, without being operated by theoptical amplification function part. Because of this feature, even whenthe optical fiber is removed in order to replace the optical add-dropfunction part in the maintenance operation, there is no impact on theoptical signals that are not involved in the maintenance operation atall as the laser safety is not operated in the optical amplificationfunction part. In addition, it is possible to eliminate the risk ofdamaging the eyes by directly seeing high power output light during themaintenance operation.

Embodiment 2

A second embodiment will be described with reference to FIG. 6. Here,FIG. 6 is a block diagram showing a principal part of an opticaladd-drop multiplexer. The differences from the optical add-dropmultiplexer shown in FIG. 4 are that the optical termination function ofthe optical add-drop function part 201-2 is realized by a variableoptical attenuator 270 instead of by the optical switch 209 and theoptical terminator 406, and that the optical switch selection circuit405 of the laser safety 220 is replaced with a variable opticalattenuator control circuit 407 of a laser safety 230.

The operation principle itself is substantially the same as in the firstembodiment. Only the difference is that when the reflection warning isdetected, the signal light is terminated by increasing the attenuationamount of the variable optical attenuator 270 to about 20 dB at once,instead of being terminated by switching the optical switch.

When a thermo-optic variable optical attenuator is used as the variableoptical attenuator 270, the attenuation amount change time, t2-t1, inthe variable optical attenuator is about 50 ms. Assuming that theprotection time tl of the reflection warning detection in the opticaladd-drop function part is 5 ms, the time needed for the optical add-dropfunction part to move to the laser safety operation is 55 ms.Accordingly it is necessary to determine the protection time t3 of thereflection warning detection in the optical amplification function partto about 550 ms which is about ten times larger than 55 ms. Incidentally550 ms is a sufficiently short time as an exposure time of the eyes tothe class 3B laser. The variable optical attenuator 270 can be areflected light mask part for preventing the reflected light from beingseen by the optical amplification function part 202-2.

Also according to the embodiment, the laser safety can be solelycontrolled by the optical add-drop function part, without beingcontrolled by the optical amplification function part. Because of thisfeature, even when the optical fiber is removed in order to replace theoptical add-drop function part in the maintenance operation, there is noimpact on the optical signals that are not involved in the maintenanceoperation as the laser safety is not operated in the opticalamplification function part. In addition, it is possible to eliminatethe risk of damaging the eyes by directly seeing high power output lightduring the maintenance operation.

As compared to the first embodiment, the advantage of the secondembodiment is that the transmitted light is not perfectly blocked but isonly attenuated by 20 dB in the termination by the variable opticalattenuator 270. Because the transmitted light is not perfectly blockedas described above, the refection warning recovery can be detected whenthe optical fiber, which has been removed during the maintenance, isreturned to the original position. Thus, unlike the first embodiment, itis possible to automatically return to the normal operation without theneed for operator operation, when the maintenance operation iscompleted.

According to the present invention, it is possible to provide opticaltransmission equipment and an optical add-drop multiplexer that have noimpact on a main signal without being involved in a maintenanceoperation for package replacement necessary due to a failure in theoptical transmission equipment or in the optical add-drop multiplexer.

1. Optical transmission equipment comprising: an optical amplificationfunction part; and an optical drop function part, wherein the opticalamplification function part includes: an optical amplifier connected toa transmission line to amplify an optical signal received from thetransmission line; and a first laser safety part for controlling saidoptical amplifier based on the power of output light of the opticalamplifier and on the power of reflected light to said optical amplifier,and said optical add function part includes: an optical couplerconnected to the optical amplification function part to split theoptical signal received from the optical amplification function part; anoptical demultiplexer for wavelength demultiplexing a first outputoptical signal of the optical coupler; a reflected light mask part forreceiving a second output optical signal said the optical coupler as aninput; and a second laser safety part for controlling said reflectedlight mask part based on the power of output light of said reflectedlight mask part and on the power of reflected light to said reflectedlight mask part.
 2. The optical transmission equipment according toclaim 1, wherein said reflected light mask part is formed by acombination of an optical switch and an optical terminator.
 3. Theoptical transmission equipment according to claim 1, wherein saidreflected light mask part is a variable optical attenuator.
 4. Theoptical transmission equipment according to claim 3, wherein said secondlaser safety part performs recovery by monitoring leaked light of saidtransmitted light of said variable optical attenuator.
 5. The opticaltransmission equipment according to claim 1, wherein said protectiontime of said first laser safety part is longer than said protection timeof said second laser safety part.
 6. An optical add-drop multiplexercomprising: an optical amplification function part; an optical dropfunction part; an optical add function part; and a second opticalamplifier connected to said optical add function part to amplify anoptical signal received from said optical add function part, whereinsaid optical amplification function part includes: a first opticalamplifier connected to a transmission line to amplify an optical signalreceived from the transmission line; and a first laser safety part forcontrolling said first optical amplifier based on the power of outputlight of said first optical amplifier and on the power of reflectedlight to said first optical amplifier, the optical drop function partincludes: an optical coupler connected to said optical amplificationfunction part to split the optical signal received from the opticalamplification function part; a first optical demultiplexer forwavelength demultiplexing a first output optical signal of the opticalcoupler; a reflected light mask part for receiving a second outputoptical signal of said optical coupler as an input; and a second lasersafety part for controlling said reflected light mask part based on thepower of output light of said reflected light mask part and on the powerof reflected light to said reflected light mask part, and said opticaladd function part includes: a second optical demultiplexer connected tosaid optical drop function part to wavelength demultiplex the opticalsignal received from said optical drop function part; a plurality ofoptical switches for receiving the outputs of the second opticaldemultiplexer and receiving from an add optical port, as inputs; and anoptical multiplexer for wavelength multiplexing the outputs of theplurality of optical switches.
 7. An optical add-drop multiplexercomprising: a reception optical amplification part including a firstoptical amplifier for amplifying received wavelength multiplexed light;an optical drop part for wavelength demultiplexing a portion of saidwavelength multiplexed light; an optical add part for replacing thewavelength demultiplexed signals of the other portion of said wavelengthmultiplexed light; and a transmission optical amplification partincluding a second optical amplifier for amplifying the wavelengthmultiplexed light from the optical add part, wherein said receptionoptical amplification part has an output portion provided with a firstlaser safety part for controlling said first optical amplifier bymonitoring reflected light, and said optical drop part has an outputportion provided with a reflected light mask part for monitoring andmasking reflected light, and with a second laser safety part forcontrolling the reflected light mask part.
 8. The optical add-dropmultiplexer according to claim 6, wherein said reflected light mask partis formed by a combination of the optical switches and the opticalterminator, or is the variable optical attenuator.
 9. The opticaladd-drop multiplexer according to claim 7, wherein said reflected lightmask part is formed by a combination of the optical switches and theoptical terminator, or is the variable optical attenuator.
 10. Theoptical add-drop multiplexer according to claim 6, wherein saidprotection time of the first laser safety part is longer than theprotection time of the second laser safety part.
 11. The opticaladd-drop multiplexer according to claim 7, wherein said protection timeof the first laser safety part is longer than the protection time of thesecond laser safety part.